Thanks for the reply. I tried that including some other changes in the source file, but I am stuck at 13 slots maximum no matter what I do.
How do I change the following in such a way that it can exceed 13 slots?
https://github.com/rathena/rathena/blob/master/src/map/clif.c#L6146
/// Opens the shop creation menu (ZC_OPENSTORE).
/// 012d <num>.W
/// num:
/// number of allowed item slots
void clif_openvendingreq(struct map_session_data* sd, int num)
{
int fd;
nullpo_retv(sd);
fd = sd->fd;
WFIFOHEAD(fd,packet_len(0x12d));
WFIFOW(fd,0) = 0x12d;
WFIFOW(fd,2) = num;
WFIFOSET(fd,packet_len(0x12d));
}